1. Field of the Invention
The invention relates to microwave cavities utilized as filters, duplexers, and transmitter combiners, and more particularly, to devices which contain more than one standing wave pattern in a single volume.
2. Description of the Prior Art
Tuned cavities of various types have been used in various high frequency communications applications for many years. Two common types of cavities include coaxial cavities and square prism filters. Such cavities are commonly connected in well known configurations to provide bandpass filters, notch filters, composite bandpass/notch reject filters and combiners (including duplexers and transmitter-combiners). Combiners as defined herein, are bidirectional devices which allow two or more bidirectional radio systems to operate on a single transmission line and antenna. Tuned filters, composite filters, and combiners of the prior art must be constructed by connecting the above described types of tuned cavities together by means of transmission lines and junction devices. Unfortunately, for commonly used communications bands such as the 450-470 and 850-870 megahertz bands which are widely used for mobile communications systems, prior tuned cavity systems are unduly large and bulky. For example, high Q coaxial tuned cavities for the 450-470 bands is approximately three feet in height and approximately one foot in diameter. A rather large number of such bulky tuned cavities may be required for a particular radio system installation. Consequently, composite tuned filter and combiner systems for the above bands are very bulky and are also unduly expensive. Obviously, the problem is more acute for the 150 megahertz band.
Due to the popularity of mobile radio communications systems in major metropolitan areas, and also due to the bulkiness of the tuned cavity filter and combiner systems of the prior art, there is an acute scarcity of optimum antenna locations. Such antenna locations are typically situated at the tops of the few tallest buildings in a particular metropolitan area. Consequently, antenna space and space for storing associated tuned filter and combiner systems is at a premium, and rental for such space is very costly.
Accordingly, it is a primary object of the invention to provide a tuned cavity system which is substantially less expensive to build than tuned cavity systems of the prior art.
Another object of the invention is to provide a tuned cavity system capable of performing substantially the same function as prior art tuned cavity systems, which tuned cavity system occupies substantially less space than tuned cavity systems of the prior art.
Still another object of the invention is to provide a tuned cavity device which can be used as a building block for tuned cavity systems which are less expensive and bulky than tuned cavity systems of the prior art.
A major reason that tuned cavity systems of the prior art are so bulky is that a large number of separate tuned cavities are required to construct composite tuned filter and/or combiner systems, since the tuned cavities of the prior art each contain only one resonant standing wave pattern.
Accordingly, an object of the invention is to provide a tuned cavity device which is capable of containing more than one resonant standing wave pattern in a single volume.
When the above mentioned tuned cavity filter and/or combiner systems are constructed by connecting individual coaxial cavities or square prism filters together by means of cables, it is necessary that cable lengths be very precisely cut (i.e., to exactly half wave lengths) in order to construct a combiner. This requirement adds additional costs, and requires services of skilled technicians when additional channels are added to a pre-existing system.
Accordingly, yet another object of the invention is to provide a combiner system without the requirement that separate tuned cavities be connected together by means of precisely cut cables.
Yet still another object of the invention is to provide a tuned cavity system which overcomes the above mentioned shortcomings of prior art tuned cavity systems and components, and which has performance characteristics including acceptably high Q, low insertion loss, and low channel separation such that the tuned cavity system can be used in place of prior tuned cavity systems in state of the art radio communication systems.
A novelty search directed to the present invention uncovered the following U.S. Pat. Nos.: 2,044,413, 2,250,308, 2,400,777, 2,477,581, 2,530,603, 2,894,225, 2,943,284, 3,247,474, 3,529,235, 3,735,289, 3,790,905, 3,851,131, 3,876,963, 3,882,434, 4,028,652, 4,034,319, 4,060,778, 4,060,779.
The state of the art is further indicated by the publications "Cavities and/or Ferrites: Their Practical Use in Combiners and Multicouplers" (Technical Bulletin No. 10419); "Application and Theory of Cavities in Duplexers" (Technical Bulletin No. 91001); and, "Low Loss Closely Spaced Multi-Transmitter Combiners", all by Ray Trott and all published by Decibel Products, Inc., of Dallas, Tex.